Until recently, a commonly used and fairly inexpensive device for small to medium sized image displays was the cathode-ray tube (CRT) monitor. However, as screen sizes increases with CRT monitors, the curvature of CRT screens also increases. In addition, the large size CRT monitors can become bulky and heavy. Energy consumption and electromagnetic radiation can also increase substantially with larger sized CRT monitors. These drawbacks have led to limited deployment in large size image display applications.
In recent years, flat panel display devices, such as liquid crystal display (LCD) and Plasma devices, have become increasingly popular. Flat panel displays can offer relatively thin packages (about 7.5˜10 cm, for example) that can be placed directly on a wall. However, such devices can be heavy and require special mounts and strengthened wall structures to hang large size flat panel devices. The brightness of flat panel displays is normally low (˜300 cd/m2 range) and the prices (usually measured by a ratio of $/diagonal length) are often high.
A high brightness alternative for large screen displays is a light emitting diode (LED) array device. In such a device a matrix of LEDs can form a large-size, bright, outdoor display. Although such devices can be attractive for applications such as outdoor advertisements, the LED arrays have a fixed (large) pixel size due to structural dimensions of the LEDs. These LED arrays are generally not suitable for indoor or portable presentation applications.
The above-mentioned display devices have pixel generation mechanisms built-in to the screens and are sometimes called “active screen” displays. Alternatives to “active screen” display devices include projection displays. Screens in projection displays are generally “passive”, meaning that images generated on the screens originate from optical projection engines placed at a distance away from the screen. There are essentially two kinds of projection display designs: rear projection and frontal projection. Rear projection displays place the projection engine and viewers on the opposite side of the light transmitting screen. In contrast, the projection engine and viewers are located on the same side of a light reflective screen for frontal projection displays.
In traditional projection display system designs, whether rear projection or frontal projection, a certain amount of space is required to accommodate a light path of the image projection to be able to expand to a large screen size without any occlusion by any internal structures or external objects. In the case of rear-projection displays, mirrors are often used to fold optical projection paths to reduce the thickness of the display package. Despite folding optical paths, the enclosure of rear projection systems can still be large and bulky. The need for space behind the display screen can preclude use of such devices as wall-mounted displays.
For frontal projection displays, traditional optical designs still require long stand-off distance. With frontal projection systems, there is typically a large “forbidden zone” between a projector and the screen. Presenters or objects cannot enter this zone without occluding the projected image on the screen. In addition, entrance into this zone can subject a person to direct viewing of the bright light from the projector, which can cause temporary blindness or dizziness. The long stand-off distances required for typical frontal projection displays can prevent a presenter from getting close to the screen or interacting with images displayed on the screen. This drawback, among others, has prevented frontal projection displays from being used in many mass commercial applications, such as small office meetings, shop window advertisements, airplane/bus/train cabin displays, portable/mobile projection systems, video gaming, and virtual reality environments. In addition, frontal projection displays are often subject to image distortion, can require a large room in which to use the system, and be difficult to set up.
Certain advances have been made in extant technologies for reducing the standoff distance of projection displays. To date, most efforts toward reducing the standoff distance of projection displays have focused on the development wide field of view (FOV) optical lens systems. Such systems are often called “short throw lenses” which can achieve shorter standoff distances. The optical designs of short throw lenses are all based on traditional, rotationally-symmetric optics, similar to the design of a fish-eye lens. These lenses can be relatively easy to make and to model mathematically. The surface shape of a rotationally symmetric lens can be created from rotating a plane curve about a chosen line, which will serve as the optical axis of the lens or mirror. There are an infinite number of possible shape designs of rotationally symmetric lenses with different selections of plane curve shape and rotating axis. This design freedom can be used to achieve certain prescribed properties for an optical system.
Short throw lenses use rotationally symmetric optics designed to rapidly expand light beams to obtain shorter standoff distances. Short throw lenses feature a very short focal length which, when used as a primary projection optic or as an attachment to existing projection optics, enables a projector to generate a large size image over a much shorter distance. To achieve shorter standoff distances, short throw lenses typically consist of multiple lenses. Use of multistage lenses, however, can introduce strong optical aberrations, which in turn require additional lenses for correction. As a result, short throw lenses can be complex to build, bulky in size and weight, and expensive in cost. Current short throw lenses are not very suitable for compact display systems and often cost as much or more than the projector in which the lens is used. Short throw lenses also typically exhibit poor performance. In addition to causing optical aberrations, such lenses also can only reduce the standoff distance by approximately ⅓ to ½ of the original projection distance without the lens. For a projector with a 2˜4 meter standoff distance, short through lenses may be able to reduce the distance maximally to 1˜2 meters, which is still insufficient for many applications. Short throw lenses also discourage user interaction since the placement of a projector may still interfere with the line of sight of the viewers or the presenter due to the traditional optical design approaches and the still rather substantial standoff distance even with the reduction by the short throw lens.